This invention relates to a method for making semiconductor devices and more particularly to steps for selectively heating to selectively alter discrete surface areas of a semiconductor substrate.
Almost all present day manufacturing processes for making transistors, light emitting diodes, Hall elements, integrated circuits and the like include many photolithographic mask making steps. Dopant diffusions are selectively effected at the semiconductor substrate surface through the mask. The mask is made by first growing an oxide layer over the substrate surface and coating the surface in a wet process with a photoresist, exposing the photoresist to a pattern of light, applying a liquid etchant to selectively remove the exposed (or unexposed) photoresist areas, and wet etching through the photoresist mask the portions of the oxide layer not covered by photoresist. After use the photoresist mask is dissolved and washed away in other liquids. These wet steps are a source of contaminants, and with the associated photolithography often represent a major portion of the entire manufacturing process.
Recently, there have been numerous proposals for selectively doping by laser irradiation of a semiconductor substrate either using a broad fixed laser beam and a contact laser-beam mask or using a fine moving laser beam (beam writing). The beam writing method is substantially slower because the discrete areas of the substrate are sequentially irradiated. The contact resin (photoresist) mask of the formerly mentioned method is made by wet photolithographic steps.
On the other hand, it has been suggested to use a laser in the making of photoresist masks themselves, whereby the laser beam passes through a separate remote mask and is optically reduced before illuminating the photoresist. The optical reduction decreases the amount of laser energy at the remote mask. However, this suggested process still requires removal of the photoresist mask from the substrate, e.g. by the dissolution and washing away of the photoresist and rinsing, and requires regrowth of a new oxide for the next step.
It is an object of this invention to provide a simpler method for making a semiconductor device including steps for selectively treating discrete surface areas of a semiconductor substrate using fewer steps and mainly dry steps.
It is a further object of this invention to provide such steps that effect selective doping of the substrate discrete areas.
It is yet a further object of this invention to provide such steps that effect epitaxial growth at the substrate discrete areas.
It is still a further object of this invention to selectively heat and melt portions of a semiconductor substrate surface with a laser beam having been patterned through a large remote mask.